This 42 kDa endo-/exonuclease (flap endonuclease and 5'-exonuclease) FEN-1 is important in DNA replication and damage repair. We conducted preliminary solution scattering experiments in the presence and absence of Mg2+ on the wild type FEN-1 and a site-directed mutant whose repair function is inactivated. Although the high-flux synchrotron beam allowed us to study these proteins at fairly low concentrations (as low as 1 mg/ml), our preliminary results suggest that the proteins needs to be measured at a rather low temperature (around 5 !C) instead of 15 !C where most of the preliminary measurements were done this time. Further studies will be performed by submitting a separate experimental proposal. Parallel neutron scattering studies are in progress.